1. Field of the Invention
The present invention relates to a method of producing a mechanical component with superior fatigue strength, such as a rocker arm used in a valve operating mechanism of an engine, through a process of casting ferrous alloy into an untreated mechanical component.
2. Description of the Prior Art
In the field of an overhead-camshaft engine, there has been proposed to use a roller rocker arm assembly which comprises a swingable rocker arm having a roller mounted thereon for transmitting rotary movements of each of cams provided on a camshaft to an intake or exhaust valve with the object of reducing friction resistance in a valve operating mechanism and thereby improving fuel consumption. In the valve operating mechanism which contains the roller rocker arm assembly and is provided in the overhead-camshaft engine, one end portion of the rocker arm constituting the roller rocker arm assembly is supported by a supporting portion provided on a cylinder head, such as a concave acceptor formed at the top of a hydraulic lash adjustor (HLA) fixed to the cylinder head, the other end portion of the rocker arm comes into contact with the top of a stem of the intake or exhaust valve, and the roller which is mounted on the middle portion of the rocker arm comes into contact with the cam provided on the camshaft. When the engine is operating, the roller of the roller rocker arm assembly is pushed down and rotated by the cam in accordance with the rotation of the camshaft and thereby the rocker arm is swung with a fulcrum positioned at the supporting portion provided on the cylinder head to transmit rotary movements of the cam to the intake or exhaust valve.
The rocker arm which constitutes the roller rocker arm assembly used as mentioned above in the valve operating mechanism of the overhead-camshaft engine is preferably required to have, as its mechanical properties, superior fatigue strength in its entirety, superior abrasion resistance particularly at the end portion thereof supported by the concave acceptor formed at the top of the HLA and another end portion thereof caused to come into contact with the top of the stem of the intake or exhaust valve, and superior accuracy in its dimension. Consequently, it has been also proposed to make the rocker arm of cement steel or ferrous alloy with superior abrasion resistance, such as standardized by the Japanese Industrial Standard JIS - G 4404 to be referred to as SKD61, through a lost-wax casting process.
However, when the rocker arm is made of cement steel through the lost-wax casting process, it is feared that melted cement steel cannot flow smoothly in a casting mold formed through a lost-wax process in process of casting so that a rocker arm obtained through a casting process is provided therein with a number of ingot pipings and thereby deteriorated in mechanical strength. For the purpose of preventing the rocker arm from having the ingot pipings, it is considered to carry out dead head in the casting process. However, in the case of the casting process for obtaining relative small casting works such as the rocker arms, generally, a plurality of casting molds are provided to be supplied with melted metallic material for casting simultaneously so as to produce a plurality of casting works at the same time, and therefore it is practically difficult to make the dead head effective actually to each of the casting molds.
Further, when the rocker arm is made of ferrous alloy such as standardized to be referred to as SKD61 through the lost-wax casting process, it is feared that a rocker arm obtained through casting process is provided with silicon-inclusions, oxygen-inclusions or the like caused to arise in grain boundaries contained in a part of a matrix structure of the ferrous alloy close to the surface thereof and thereby comes to fatigue failure or fatigue fracture resulted from the silicon-inclusions, oxygen-inclusions or the like positioned to be close to the surface when external force acts on the rocker arm.